ITMI972733A1 - POLYMER MEMBRANE FUEL CELL OPERATING AT TEMPERATURE ABOVE 100 ° C - Google Patents

Description

DESCRIPTION OF INDUSTRIAL INVENTION

Fuel cells are apparatuses in which the free reaction energy released by the combination of a fuel (for example methanol, ethanol, hydrogen, or mixtures containing the latter) with a comburent (for example pure oxygen, air, chlorine or bromine ) is not completely degraded to thermal energy, but converted into electrical energy in the form of direct current. In these apparatuses, the fuel is fed to the anode, which assumes negative polarity, and the comburent is fed to the cathode, which, vice versa, assumes positive polarity. The generation of electricity in fuel cells is of great interest due to the high efficiency of use of the fuels used, as the process does not suffer from the limitations of the Carnet cycle, and due to the low environmental impact, in terms of harmful emissions and noises. If pure hydrogen is chosen as a fuel, this environmental impact is close to zero.

Fuel cells can be schematically classified into different types, essentially characterized by the type of electrolyte that separates the anodic and cathodic compartments, and consequently by the temperature range at which they can typically operate; this type of classification directly reflects on the use that can be achieved or envisaged for these systems. In particular, fuel cells that operate at high temperatures, that is to say above 200 ° C, are now emerging as an alternative source of electricity in large power plants, also due to the interesting possibilities of cogeneration allowed by the high thermal level. On the other hand, the type of cell that appears most interesting in the field of small and medium-sized electrical generation, both for stationary and mobile applications (for example the traction of motor vehicles) uses a polymeric membrane as an electrolyte. ion exchange, whose use at temperatures above 100 ° C is traditionally prevented by the need to maintain a high degree of hydration inside. The high energy and power density associated with the use of solid polymeric electrolytes and the speed of start-up and running of the fuel cells, which use them, make membrane fuel cells much more competitive than any competitor. for such type of applications.

However, the limitation of the thermal level allowed by ion exchange membranes constitutes an important limitation to their full success on the market, above all due to the lack of flexibility of the fuel fed at temperatures below 100 ° C. The availability of pure hydrogen, in fact, is limited to a selected niche of applications in which this fuel is present as a by-product, as in the case of sodium chloride electrolysis plants. The market situation of the present and the future, at least in the medium term, requires the use of fuels for which an already existing distribution network exists, or which offer a good ease of transport, as in the case of liquid fuels.

Among the most promising fuels to allow the affirmation of fuel cells in the generation of electricity with low environmental impact, it is necessary to mention light alcohols (methanol, ethanol), i.e. hydrogen from water vapor reforming or oxidation partial of widely available precursors such as natural gas, liquid hydrocarbons (petrol, diesel, etc.) or of the same light alcohols in the event that direct oxidation is not sufficiently efficient. However, none of these fuels is reasonably compatible with the present polymer membrane fuel cell technology, operating at low temperature. Light alcohols present two types of currently insurmountable problems: on the one hand, their oxidation kinetics at low temperature is completely insufficient to make them competitive, in terms of efficiency, with respect to simple combustion processes, even if the most efficient are used. noble metal-based catalysts; on the other hand, the high permeability of commercially available membranes with respect to alcohols causes a partial depolarization of the process, resulting in a further loss of electrical efficiency. In the case of hydrogen coming from the conversion of natural gas, alcohols or fossil fuels, the problem represented by the inevitable presence of carbon monoxide is well known, of which simple traces are sufficient to significantly penalize the operation of the anodic catalysts currently known at commonly used temperatures. employed. Both the problem of the oxidation kinetics of alcohols on noble metals, and the phenomena of poisoning of the latter due to exposure to very limited quantities of carbon monoxide, can be virtually eliminated by increasing the operating temperature above 130 ° C. Commercial membranes for fuel cells really developed to the point of allowing their effective use in industrial applications are 1 made up of perfluorocarbosulfonic acids (e.g. Nafion®), for which the possibility of operating at medium temperature is prevented for two orders of reasons; the first thermal threshold, set at 10CPC, cannot be crossed due to the difficulty of maintaining the water balance of the system, especially when operating with low pressure gaseous reagents (a condition imposed for most practical applications in order to maximize the efficiency of system). The second thermal threshold, set at about 130X, corresponds to the beginning of the phase transition phenomena for most of the perfluorocarbosulfonic acids; above this temperature, for prolonged operating times, crystalline phases begin to segregate inside the polymer, which create inhomogeneities which reflect on the properties of electrical conduction and mechanical stability. The inhomogeneity of the current distribution causes, in these conditions, a bad thermal distribution which causes the destruction of the polymer in the higher temperature areas ("hot spots").

An improvement in the humidification conditions of perfluorocarbosulfonic membranes has been proposed in U.S. Pat. 5,523,181; in the case of fuel cells operating with hydrogen and oxygen or with hydrogen and air, the dispersion of very fine particles of silica gel inside the polymers in question favors their water retention, allowing the operation of fuel cells with supply reduced humidity in the flow of gaseous reactants or, in the most favorable cases, the maintenance of the water balance of the system without adding humidity from the outside, by means of only the water generated by the reaction. This invention, while not allowing the operation of fuel cells above 100 ° C in most of the process conditions required by industry (for example, feeding low pressure reagents), and while not facing the problem of the thermal threshold at which the phase transition phenomena of the polymers making up the ion exchange membranes begin, makes it possible to considerably simplify the system around fuel cells operating at low temperatures, and constitutes an important step forward towards achieving the objectives that the present invention aims.

The invention in question is aimed at operating medium temperature polymer membrane fuel cells (100 - 160 ° C), with a wide flexibility in the choice of the fuel to be fed to the anode (light alcohols, hydrogen from steam reforming or from partial oxidation of alcohols or hydrocarbon-based fossil fuels, or natural gas).

The operation of conventional ion exchange polymers under the conditions mentioned is made possible through the substantial implementation of the invention described in U.S. 5,523,181, sometimes cited in the specialized literature as "Stonehart membrane" (for example, Fuel Celi Seminar 1996 Proc., Page 591 ff.), And obtainable by dispersion of Si02 (0.01 -50% by weight) in a suspension in liquid phase of the ion exchange polymer, with consequent redeposition of the mixture obtained in a film with a controlled thickness (typically from 3 to 200 micrometers depending on the applications), using a Petri dish or similar techniques.

The implementation object of our invention consists of a heat treatment at a temperature above 130 ° C, and preferably above 150 ° C, for a limited period of time from 1 to 60 minutes, preferably from 3 to 15 minutes, so as to inducing a controlled phase transition of part of the polymer under examination; obtaining the optimal degree of conversion from the amorphous to the crystalline phase can be controlled by X-ray diffraction during the execution of the heat treatment.

It has surprisingly been noted that, following this treatment, the mechanical properties of the film are considerably increased, and that operation in fuel cells at temperatures between 100 and 160 ° C is possible for prolonged periods of time, without the occurrence of phenomena of dehydration of any kind and without the appearance of further uncontrolled phase transition phenomena.

The operation of fuel cells at temperatures above 130 ° C, in particular, allows considerable flexibility in terms of fuel fed to the anode, allowing both the suppression of phenomena of poisoning of noble metal-based catalysts by monoxide of carbon and other organic compounds, and the considerable increase in the oxidation kinetics of fuels such as methanol and ethanol, finally, it has been surprisingly noticed how, in the case of cells fed with light alcohols, the phenomena of permeatone by these fuels through the membrane are considerably reduced, as indirectly demonstrated by the reduced depolarization of the process under examination, whose open circuit electric voltage reaches unusually high values. The diffusion coefficient of the methanol species was also determined experimentally in real working conditions: the value obtained, equal to 1.33.10 "8 cm2 s1, is about a thousand times lower than the values usually reported in the literature (eg M.W. Verbrugge , J. Electrochem. Soc. 136. N ° 2, 1989, pp. 417-423)

The main purpose of the present invention is to provide an improved proton conduction membrane, suitable for working at temperatures above 100 ° C and with a reduced permeation to alcohols, consisting of a perfluorocarbosulfonic acid in which silica particles with a concentration by weight are incorporated comprised between 0.01 and 50% by weight, characterized in that it is subjected to a heat treatment at a temperature higher than the glass transition temperature, for a time sufficient for a partial conversion of the amorphous phase into the crystalline phase.

It is a further object of the present invention to provide an electrochemical cell with membrane where oxidation takes place at the anodic compartment of a fuel selected from the group which includes methanol, ethanol, hydrogen coming from conversion of hydrocarbons or alcohols, using the improved membrane of the invention .

The following is a detailed description of the invention in question, which refers to some representative examples of the best way to apply the invention itself, without these examples having in any way the purpose of limiting or circumscribing its use to what is expressly mentioned. .

EXAMPLE 1

In 100 grams of hydroalcoholic suspension of Nafion® (5% by weight, Aldrich) a quantity of Si02 (Aerosil 200, Degussa) equal to 3% by weight, in the form of fine particles, with particles having a diameter of less than a micrometer, was suspended ; the mixture thus obtained was subjected to stirring in an ultrasonic bath for 30 minutes. Subsequently, this mixture was deposited in a Patri disk and heated for 30 minutes at 80 ° C. The redeposited Nafion film was detached from the Petri dish by adding distilled water and left to dry for 15 hours at room temperature, then interposed between two PTFE sheets, to which a pressure of 3 bar was applied. A thermal ramp was then applied to the sample, housed in a Paar HTK-10 high temperature chamber inserted in a Philips X'Pert System PW 1830 X-ray diffractometer, increasing the temperature up to 160 ° C and controlling the transition of consequently induced phase on the polymer through the XRD emission.

In particular, the displacement of the peak emitted at 2Θ = 17 ° (CuKa) towards values of increasing 2Θ was monitored: the heat treatment was interrupted when this peak reached 2Θ = 17.8 ° (CuKa) , corresponding to a 25% conversion of the amorphous phase to the crystalline phase.

At the end of the treatment, the polymeric film, with a thickness of about 150 micrometers, was subjected to a purification cycle with 5% hydrogen peroxide and 1 M sulfuric acid.

The membrane thus obtained was cut into the desired shape, and two ELAT ™ electrodes (E-Tek, Ine.), I.e. a cathode activated with 0.3 mg / cm2 of Pt supported on carbon, and a activated anode with 0.6 mg / cm2 of 1: 1 Pt.Ru alloy supported on carbon (provided by E-Tek, Ine.); this membrane - electrode assembly was inserted in a fuel cell fed to the cathode with air at 1.5 absolute bar, and to the anode with pure hydrogen at the same pressure; the temperature of humidification of the reagents and that of the cell were kept at 70 ° C. A second reference cell was equipped with the same components as the first, except for the membrane, consisting of a product marketed by Dupont under the Nafion® 115 brand, having the same thickness and the same chemical composition of the membrane inserted in the first cell, if except for the addition of silica. Also in this case the same reagents were fed as in the previous case, with the same process parameters.

The cells, having 50 cm2 of active area, were put into operation at a constant current density of 5 kA / m2 over the course of 8 hours, measuring the cell voltage continuously. In both cases, this parameter remained in a range between 690 and 700 mV, which was practically constant over time.

After an overnight stop, both cells were put into operation at a temperature of 100 ° C. with reagents wetted at 85 ° C; the other parameters were kept constant. The cell equipped with the membrane of the present invention made it possible to detect, at a current density of 5 kA / m2, a cell voltage comprised between 705 and 715 mV, constant over time over 8 hours. The cell equipped with the Nafion® 115 membrane made it possible to detect an initial voltage of 710 mV; this voltage gradually dropped to zero over the course of 30 minutes.

EXAMPLE 2

The cells of the previous example, one equipped with the membrane of the present invention, described in example 1, the other with Nafion® 115, were fed to the cathode with air at 2 absolute bar, and to the anode with a mixture coming from methane steam reforming and subsequent water shift at the same pressure; the composition of this mixture comprises 65% of hydrogen, 1% of carbon monoxide, in addition to carbon dioxide, water and other inerts. The humidification temperature of the reagents and that of the cell were maintained at 70 ° C.

It was not possible to detect any generation of electric current in the two cells.

The temperature of the two cells was subsequently increased up to 100 ° C, and that of the humidification of the reagents up to 85 ° C; the cell equipped with the commercial reference membrane Nafion® 115 showed weak signals of electric current generation, which quickly ceased within a few minutes.

The cell equipped with the membrane of the present invention generated, under these conditions, 5 kA / m2 of direct current at a voltage of 620 mV; the cell temperature was subsequently raised up to 150 ° C, and under these conditions it was possible to generate 5 kA / m2 of direct current at a voltage of between 680 and 700 mV for eight hours continuously.

EXAMPLE 3

The cell of the previous examples, equipped with the membrane of the present invention, described in example 1, was fed to the anode with a mixture of 2M methanol in water preheated to 85 ° C, and to the cathode with air at 4 absolute bar; the outlet of the anodic compartment was pressurized to 4.5 absolute bar. A reference cell was equipped with the same components as the first, except that the membrane, obtained with the same formulation of Nafion® (5% by weight, Aldrich) and Si02 (Aerosi! 200, Degussa), is not been subjected to any heat treatment. The reference cell was also put into operation under similar operating conditions. The temperature of the two cells was initially set at 85 ° C. An open circuit voltage of 950 mV was read on the cell equipped with the membrane of the present invention; on the reference cell, the open circuit voltage was 890 mV, evidently due to a more pronounced methanol permeation.

Both cells were put into operation at a current density of 2 kA / m2; on the cell equipped with the membrane of the present invention, a voltage of 260 mV was detected, progressively reduced to 240 mV over two hours and subsequently stabilized at this value over the next six hours; a voltage of 60 mV was detected on the reference cell, which gradually decreased to 30 mV over the next eight hours.

The temperature of both cells was subsequently raised to 155 ° C; under these conditions it was not possible to put the reference cell into operation, since the cell voltage dropped to zero in the space of thirty minutes, the cell equipped with the membrane of the present invention made it possible to detect, at a current density of 2 kA / m2, an initial cell voltage of 600 mV, which stabilized at 550 mV over one hour and then remained stable over the next eight hours. The current density was then brought to 500 mA / cm2, and in these conditions the detected cell voltage remained constant at 400 mV over the course of the following eight hours; during this test, a diffusion coefficient of methanol through the membrane equal to 1.33 was measured by gas chromatography at the exit of the anodic compartment. IO * cm2 s'1, three orders of magnitude lower than the values reported in the literature.

EXAMPLE 4

The two cells of example 3 were fed to the anode with a mixture of 2M ethanol in water preheated to 85 ° C, and to the cathode with air at 4 absolute bar; the outlet of the anodic compartment was pressurized to 4.5 absolute bar.

At a cell temperature of 85 ° C, open circuit voltages of 820 mV were measured for the cell equipped with the membrane of the present invention, and of 740 mV for the reference cell.

At the same temperature, it was not possible in either cell to reach a current density of 0.5 kA / m2.

The temperature of the two cells was subsequently raised to 145 ° C; under these conditions, the cell equipped with the membrane of the present invention generated 2 kA / m2 of direct current at a voltage between 460 and 490 mV during the course of eight hours of testing; the reference cell, after a few minutes of operation with an initial voltage of 435 mV at the same current density, has progressively ceased to generate current.

EXAMPLE 5

Five fuel cells, having 50 cm2 of active area, equipped with the same membrane-electrode assembly, comprising the membrane of the present invention, described in example 1, were connected in electrical series in a stack with feeding of the anodic and cathodic reagents. in parallel.

The cells were fed to the cathode with non-humidified air at 4 bar, and kept at a temperature of 150 ° C for the whole duration of the experiment; the anode was first fed with the mixture containing hydrogen and carbon monoxide of Example 2, humidified at 85 ° C, at a pressure of 4 bar; the stack was then put into operation at a current density of 5 kA / m2; over the course of two hours, the average cell voltage remained within values between 670 and 680 mV.

Subsequently, the feeding line of the anodic compartment was purified and then fed with the mixture containing water and methanol of Example 3; the current density was fixed at 2 kA / m2 for two hours, during which the mean cell voltage detected was kept within values between 530 and 550 mV.

The anodic feeding line was finally reclaimed, then fed with the mixture containing water and ethanol of Example 4; the current density was maintained at a constant value of 2 kA / m2 over the course of two hours, during which the mean cell voltage detected was kept within values between 425 and 440 mV.

The examples set out above are not intended to limit the invention to the cases explicitly indicated, but on the contrary the invention intends to cover any modification or alternative configuration that falls within the spirit and scope of the invention itself as set out in the following claims.

Claims (10)

CLAIMS 1. A proton conduction membrane consisting of a perfluorocarbosulfonic acid in which silica particles with a concentration by weight of between 0.01 and 50% by weight and a size of between 0.001 and 10 micrometers are incorporated, characterized by the fact that they have improved characteristics of thermal and mechanical stability obtained through a heat treatment at a temperature higher than the glass transition temperature, for a time sufficient for a partial conversion of the amorphous phase into the crystalline phase. 2. The membrane of claim 1 in the case where such heat treatment is controlled by means of an X-ray diffraction spectrometer. 3. Method of operation of an electrochemical cell with membrane where oxidation takes place in the anodic compartment of a fuel selected from the group that includes methanol, ethanol, hydrogen coming from conversion of hydrocarbons or alcohols, comprising the membrane of the riv. 1 or 2. 4. The method of rev. 3 characterized in that this cell operates at a temperature above 100 ° C. 5. The method of rev. 4 characterized in that these fuels are fed at temperatures below 100 ° C. 6. The method of claims 3, 4 or 5 characterized in that said cell is a fuel cell. 7. A membrane electrochemical cell where oxidation takes place in the anodic compartment of a fuel selected from the group which includes methanol, ethanol, hydrogen coming from conversion of hydrocarbons or alcohols, comprising the membrane of the riv. 1 or 2. 8. The cell of claim 7 characterized in that said cell operates at a temperature higher than 100 ° C. 9. The cell of claim 8 characterized in that said fuels are fed at temperatures below 100 ° C. 10. The electrochemical cell of claims 7, 8 or 9 characterized in that said cell is a fuel cell.